This invention relates to an apparatus for producing a thermoplastic resin foam.
It has been known that the thermoplastic resin foam can be effectively produced, continuously, by means of an extrusion method. (Therefore, it has been usual to use the extrusion method for this purpose.) In this case, in order to obtain a highly expanded foam, an extrudate is usually introduced immediately into a vacuum chamber connected to a die member of the extruder to enhance the expansion of the foam under reduced pressure. In such a case, the foam accomplished expansion is produced continuously from the outlet of the vacuum chamber. Therefore, it is necessary to prevent air from being introduced into the reduced pressure device through an annular gap possibly formed between an outer surface of the foam and an inner wall of an outlet hole of the vacuum chamber. In order to realize the prevention of air immigration into the vacuum chamber, U.S. Pat. No. 3,822,331 discloses either a flexible flap on an outer end wall of the chamber or a flexible ring packing provided on an inner wall of the outlet hole of the chamber which acts to prevent air from passing through an annular gap between the inner wall of the hole and the outer periphery of the extruded resin foam.
The flexible packing is of natural or synthetic rubber or soft resin and is provided so that it covers the outlet hole of the chamber except a center portion thereof through which the extruded resin passes against a resilient force acting to minimize the open area of the center portion. It has been found that such packing provided in the outlet of the chamber is insufficient to provide a satisfactory air tightness thereof. That is, during a run of foam expansion, the volume of the foam increases at one variable rate and thus the cross-sectional configuration and size of the foam are not always constant. The packing itself cannot follow these physical variations of the foam. Therefore, it has been desired to improve the air tightness of the outlet of the vacuum chamber.